Gwenlan, Claire; (2004) Jets and energy flow in photoproduction using the ZEUS detector at HERA. Doctoral thesis , UCL (University College London).

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Abstract

The hard photoproduction of jets has been studied using the ZEUS detector at HERA. The data were collected during the 1996-1997 e+p running period and correspond to an integrated luminosity of 38.6 pb-1. Events were selected with a photon-proton centre-of-mass energy in the range 134 W7P 277 GeV with photon virtualities of Q2 1 GeV2. Jets were defined using the longitudinally invariant k algorithm in the inclusive mode. Cross sections for the photoproduction of four high transverse energy jets have been measured. This is the highest order process in QCD to have been studied at HERA to date. Two kinematic regions have been defined: the inclusive and the high mass samples. The high mass sample contains only events with a four jet invariant mass satisfying m.;j 50 GeV. The data axe compared to parton shower models which resum leading logarithms to produce multi-parton final states. The inclusive distributions are found to be very sensitive to the underlying event model. Models with no simulation of the underlying event are unable to describe the shape of the distributions. Inclusion of only a soft underlying event is also disfavoured. However, the description of the data is significantly improved by inclusion of multi-parton interactions, which introduces a hard, partonic scattering component to the underlying event. The high mass cross sections are well described by the parton shower models and are relatively insensitive to the underlying event model and, consequently, are considered suitable for comparison with fixed order perturbative QCD calculations when they become available. The rate of four jet photoproduction, relative to that of three jet, shows an excess over the predictions of models which use only initial- and final-state QCD radiation to produce multi-parton final states. The data can be described by models including multi- parton interactions, which provides an additional mechanism for the production of four jets. The data from this analysis have also been used to provide constraints on the parton shower model parameters. Cross sections for the photoproduction of two jets separated by a large region in rapidity have been measured. Rapidity gap events are defined in terms of the energy flow in the region between the two highest transverse energy jets. Gap fractions, defined as the ratio of the two jet gap cross sections to the two jet inclusive cross sections, have also been measured. An excess of events with a rapidity gap, over that expected from standard QCD, is observed. This can be interpreted as evidence for the exchange of a strongly interacting colour singlet object. The data are well described by the leading logarithmic approximation BFKL pomeron with a choice of c.;re' = af271' = 0.18 and UJQ = 0.48. Taking into account the constraints imposed on the underlying event model from the multi-jets analysis, an estimate of S = 65 - 70% has been estimated for the gap survival probability.

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